Move Image-specific functions from RenderDevice to Image class
Don't use "Surface" in RenderDevice function names

The following changes have been made:
Image::scaleSurface() -> Image::resize()
RenderDevice::loadGraphicSurface() -> RenderDevice::loadImage()
RenderDevice::createAlphaSurface() -> RenderDevice::createImage()
Rename Renderable's "sprite" variable to "image"
Merge pull request #1081 from dorkster/renderdevice

More RenderDevice fixes